MOUNT VERNON, Ohio (AP) — Ohio health officials say a measles outbreak that began among Amish who had traveled to the Philippines now involves 21 cases confirmed over three counties.

The state Department of Health reports the number of cases in Knox County in north-central Ohio has increased to 19. Nearby Richland and Wayne counties each have one confirmed case of the highly contagious respiratory illness.

Officials say the outbreak began with unvaccinated travelers who visited the Philippines, which has had a measles epidemic.

Officials are urging vaccination to limit the Ohio outbreak. In Morrow County, adjacent to Knox County, health officials plan a free immunization clinic Wednesday evening for anyone who hasn’t received two doses of the measles-mumps-rubella vaccine.

Measles is caused by a virus. The symptoms include fevers, coughs, rashes and pink eye.